Diols are important starting materials for a variety of industrial products such as adhesives, lubricants, polyurethanes, and coatings. Diols can be produced by various methods, including hydrogenolysis of sorbitol, glycerin and glucose. Hydrogenolysis of sorbitol can produce an assortment of alcohols, including monoalcohols of 1-4 carbons, and polyols, such as various diols of 2-4 carbons, and glycerol, as well as organic salts. As sorbitol is usually not completely converted to lower alcohols in a hydrogenolysis process, it can also be present in the resultant reaction mixture. Depending on the reaction conditions and the catalysts used in the hydrogenolysis of sorbitol, different products may be obtained, and the relative amounts of the products may also vary.
Separating diols from a mixture comprising diols and polyols (e.g., sorbitol and glycerol) can be technically challenging. Conventional distillation methods used in the industry typically require a distillation temperature of 140° C. or greater, and can achieve only around 80 to 95% yield of diols from the mixture resulting from sorbitol hydrogenolysis. The high temperature and prolonged distillation time (usually tens of minutes) can result in a substantial extent of polymerization of polyols (e.g., glycerol and sorbitol). The polymerization of these polyols, along with an increase in the concentration of the organic salts, in the distillation process can significantly raise the viscosity of the mixture, which can make the heat and mass transport in a reaction system for sorbitol hydrogenolysis more difficult. The polymerization also decreases the amount of glycerol and sorbitol reusable for the catalytic hydrogenolysis process. Glycerol can, under certain conditions of sorbitol hydrogenolysis, be converted to diols similar to those produced by hydrogenolysis of sorbitol.
Certain evaporators permit efficient separation of the components of a liquid mixture where the components of the liquid mixture are heat sensitive, e.g., susceptible to degradation or polymerization upon exposure to excessive heat. For example, a wiped-film evaporator is equipped with wiping elements for distributing a thin layer of liquid on an evaporation surface. This design greatly increases the area of the effective evaporation surface and shortens the required period of heat exposure of the liquid passing over the evaporation surface (also known as the residence time). Another type of evaporator, short-path evaporator (or molecular evaporator), has a condenser housed within its evaporator body and placed within a very short distance from an evaporation surface. Compared with a wiped-film evaporator, a molecular evaporator can typically provide higher vacuum for distillation partly because vapor leaving a liquid mixture that flows through the evaporation surface is promptly “captured” by a nearby condenser, thereby avoiding a buildup of pressure of the gas phase in a reactor.
U.S. Pat. No. 5,710,350 to Jeromin et al. (“the '350 patent”) discloses a process for separating diglycerol from glycerol in polyglycerol synthesis. The process is based on condensation of diglycerol through successive steps of glycerol removal: glycerol is first separated from diglycerol, which remains in a first bottom product, in a wiped-film or short-path distillation zone at a pressure of 0.5 to 5 mbar, then the first bottom product (containing diglycerol) is further distilled in a short path distillation zone at a pressure of 0.05 to 3 mbar. The '350 patent states that about 90% or greater of diglycerol remains in the first bottom product.
Accordingly, there remains a need to develop novel and effective methods for separating diols from a mixture comprising diols and polyols to produce diols in high yields while avoiding the disadvantages of conventional distillation-based separation methods.